1. Field of the Invention
The present invention relates to an inkjet head for performing print on a print medium by ejecting ink according to an inkjet system and a substrate for the inkjet head.
2. Description of the Related Art
Depending on various inkjet systems, there are various types of inkjet heads (hereinafter, also referred to as inkjet print heads or print heads) used in printing apparatuses (hereinafter, also referred to as printers) that make print by applying ink onto print media such as printing paper. In one of the systems, an inkjet print head heats ink to form bubbles by using, as an energy generating element (also called a print element), a heater element (also called a heater) configured of a heating resistor that generates heat in response to conduction, and makes print by using pressure generated from the bubbles thus formed. This enables highly fine printing at high speed because it is easy to highly accurately manufacture a substrate for a print head substrate in which a large number of heater elements, wirings and the like are arranged with a high density. Moreover, this leads to achievement of further size reduction of a print head and accordingly a printer using the print head.
If all of a large number of heater elements thus arranged are driven at one time, a large amount of current flows into the print head instantaneously. To avoid this situation, several tens to several hundreds of heater elements are divided into several blocks, and the blocks of heater elements are driven at slightly different timings. By reducing the number of heater elements driven at one time as described above, the total amount of instantaneously flowing current is limited to a predetermined low level. Moreover, heater element drive circuits for driving a large number of heater elements are incorporated in the print head. This configuration is intended to prevent an increase in the number of wirings between the print head and a printer in which the print head is mounted. In widely used configurations, these drive circuits are built in a Si (silicon) wafer used as a substrate for forming the heater elements.
There are various configurations of the drive circuits. Here, a configuration disclosed in Japanese Patent Laid-Open No. 2002-307685 is described as a typical configuration.
The driving of each heater element is controlled in accordance with a block control signal (BLKn) indicating a number of a block to which the heater element belongs, and a print signal (DATA) corresponding to the block control signal (DATA). As the block control signal (BLKn), binary data obtained by encoding a block number are used. Here, a value calculated by dividing the total number of heater elements (T) by the total number of blocks (N) indicates the number of heater elements (M=T/N) drivable at one time in one driving operation. Moreover, one bit of the print data corresponds to one heater element. If the print data are transferred in an amount corresponding to the number of heater elements that are simultaneously driven at one driving operation in a time division driving, M represents the number of bits of the print signals (DATA) for driving the heater elements simultaneously driven in one operation.
Moreover, gates and transistors are provided for the number of bits equal to the number of heater elements (T). In addition, a shift register and a latch circuit are provided for the number of bits of the block control signals (BLKn) and the number of bits of the print signals (DATA) for simultaneously driving the heater elements in one driving operation. Data composed of the print signals (DATA) and the block control signals (BLKn) are serially transferred to the shift register from a controller of printer main body. After the print signals (DATA) are aligned in the shift register, the print signals (DATA) are latched by the latch circuits. For each block, drive signals are obtained by decoding the block control signals, and the obtained drive signals and the latched print signals are inputted to the gates. In response to this input, the transistors corresponding to the heater elements of the block are driven. Incidentally, this patent document discloses that a usable type of transistors is a bipolar transistor or a field effect transistor (FET).
As for an inkjet print head, the ejection characteristics of ink ejected from ejection openings and the temperature of the substrate for the print head are closely related to each other. For this reason, detecting the substrate temperature plays an important role in the control to obtain stable ejection characteristics.
For this purpose, as described in Japanese Patent Laid-Open No. H02-258266 (1990), a conventional inkjet print head is equipped with a temperature sensor built in a substrate for a print head and thus are enabled to read the substrate temperature with high accuracy. This temperature sensor is used to control the ink ejection characteristics that vary due to heat. In addition, using monitor values of the temperature sensor, the inkjet print head forcefully suspends a printing operation for a brief period of time when the substrate temperature rises to an abnormally high degree due to an occurrence of some problem in the substrate. For example, such forceful suspension is performed when the power supply shorts out or when the heater elements are driven under the condition where no ink is left because of any factor (this driving situation is called “blank ejection”).
One known temperature sensor has a diode type configuration. Japanese Patent Laid-Open No. 2004-050637 discloses that a diode type of temperature sensor is arranged near a connection terminal (input/output pad) for transmitting and receiving print signals and other signals to and from external units.
To meet a recently-increasing demand for printing at even higher speed, the number of ejection openings and heater elements arranged in a print head has been increasing. In other words, a print head is configured to have a larger print swath by increasing the number of ejection openings and heater elements in an arranging direction (increasing a size or length of a print head). In some cases, however, such length increase causes the temperature distribution in the substrate for the print head to largely vary depending on the driving status of respective heater elements corresponding to the contents of print data or a print mode.
Since the conventional substrate for the print head is relatively small, the temperature sensor is capable of detecting the temperature with sufficiently high accuracy even when being disposed in a position not very close to an array of heater elements, that is, in a position close to a connection terminal. However, as the size of the print head increases, the distance from the temperature sensor to the array of heater elements, especially the array center, increases, thereby making it more difficult to detect the accurate temperature.
As a result, even though it is strongly desirable to dispose the temperature sensor in a position corresponding to the center of the heater element array, the conventional substrate configuration has the following problems.
In the conventional substrate configuration, logic circuits for bringing the heater elements into conduction in response to the drive signals, and logic wirings connected to the logic circuits are disposed. Of these, the logic circuits are formed as an “underlying layer” in a base plate made of Si by a semiconductor manufacturing process, and electrodes and the like for the logic circuits are formed by using an underlying wiring layer. On the other hand, the logic wirings are arranged in parallel in number corresponding to the number of heater elements. Accordingly, the number of logic wirings increases as the number of heater elements increases. Because the underlying wiring layer allows size reduction in the width of wirings and the spaces between the wirings, the underlying wiring layer is also used to form the logic wirings. When a functional element such as a diode sensor, for example, is used as the temperature sensor, the functional element is also formed by the semiconductor manufacturing process as in the case of the logic circuits. Thus, the functional element is also formed by using the underlying wiring layer. Instead, the sensor may be formed by using a wiring or the like as a resistor. In this case, the resistance value of the wiring or the like needs to be locally increased to a sufficiently high level for achieving high detection sensitivity. To form such sensor, it is also desirable to use the underlying wiring layer that allows size reduction in the width of lines and the spaces between the lines.
As described above, even if the temperature sensor is attempted to be disposed in a position corresponding to the center of the heater element array, the logic circuits and the logic wirings are located around the heater element array and therefore, the temperature sensor is disposed at an outer side of these circuits and wirings. As a result, the substrate size in a width direction is increased by an area of the temperature sensor. Consequently, problems arise that the number of substrates manufactured from a single Si wafer is reduced, thereby increasing costs for the substrates.